Sunday, 27 October 2013

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The display resolution of a digital television, computer monitor or display device is the number of distinct pixels in each dimension that can be displayed. It can be an ambiguous term especially as the displayed resolution is controlled by different factors in cathode ray tube (CRT), Flat panel display which includes Liquid crystal displays, or projection displays using fixed picture-element (pixel) arrays.

It is usually quoted as width × height, with the units in pixels: for example, "1024 × 768" means the width is 1024 pixels and the height is 768 pixels. This example would normally be spoken as "ten twenty-four by seven sixty-eight" or "ten twenty-four by seven six eight".

One use of the term “display resolution” applies to fixed-pixel-array displays such as plasma display panels (PDPs), liquid crystal displays (LCDs), digital light processing (DLP) projectors, or similar technologies, and is simply the physical number of columns and rows of pixels creating the display (e.g., 1920 × 1080). A consequence of having a fixed-grid display is that, for multi-format video inputs, all displays need a "scaling engine" (a digital video processor that includes a memory array) to match the incoming picture format to the display.

Note that for broadcast television standards the use of the word resolution here is a misnomer, though common. The term “display resolution” is usually used to mean pixel dimensions, the number of pixels in each dimension (e.g., 1920 × 1080), which does not tell anything about the pixel density of the display on which the image is actually formed: broadcast television resolution properly refers to the pixel density, the number of pixels per unit distance or area, not total number of pixels. In digital measurement, the display resolution would be given in pixels per inch. In analog measurement, if the screen is 10 inches high, then the horizontal resolution is measured across a square 10 inches wide. This is typically stated as "lines horizontal resolution, per picture height;"[1] for example, analog NTSC TVs can typically display about 340 lines of "per picture height" horizontal resolution from over-the-air sources, which is equivalent to about 440 total lines of actual picture information from left edge to right edge

Some commentators also use display resolution to indicate a range of input formats that the display's input electronics will accept and often include formats greater than the screen's native grid size even though they have to be down-scaled to match the screen's parameters (e.g., accepting a 1920 × 1080 input on a display with a native 1366 × 768 pixel array). In the case of television inputs, many manufacturers will take the input and zoom it out to "overscan" the display by as much as 5% so input resolution is not necessarily display resolution.

The eye's perception of display resolution can be affected by a number of factors – see image resolution and optical resolution. One factor is the display screen's rectangular shape, which is expressed as the ratio of the physical picture width to the physical picture height. This is known as the aspect ratio. A screen's physical aspect ratio and the individual pixels' aspect ratio may not necessarily be the same. An array of 1280 × 720 on a 16:9 display has square pixels, but an array of 1024 × 768 on a 16:9 display has rectangular pixels.

An example of pixel shape affecting "resolution" or perceived sharpness: displaying more information in a smaller area using a higher resolution makes the image much clearer or "sharper". However, most recent screen technologies are fixed at a certain resolution; making the resolution lower on these kinds of screens will greatly decrease sharpness, as an interpolation process is used to "fix" the non-native resolution input into the display's native resolution output.

While some CRT-based displays may use digital video processing that involves image scaling using memory arrays, ultimately "display resolution" in CRT-type displays is affected by different parameters such as spot size and focus, astigmatic effects in the display corners, the color phosphor pitch shadow mask (such as Trinitron) in color displays, and the video bandwidth

systems use interlaced video scanning with two sequential scans called fields (50 PAL or 60 NTSC fields per second), one with the odd numbered scan lines, the other with the even numbered scan lines to give a complete picture or frame (25 or 30 frames per second). This is done to save transmission bandwidth but a consequence is that in picture tube (CRT) displays, the full vertical resolution cannot be realized. For example, the maximum detail in the vertical direction would be for adjacent lines to be alternately black then white. This is not as great a problem in a progressive video display but an interlace display will have an unacceptable flicker at the slower frame rate. This is why interlace is unacceptable for fine detail such as computer word processing or spreadsheets. For television it means that if the picture is intended for interlace displays the picture must be vertically filtered to remove this objectionable flicker with a reduction of vertical resolution. According to the Kell factor the reduction is to about 85%, so a 576 line PAL interlace display only has about 480 lines vertical resolution, and a 486 line NTSC interlace display has a resolution of approximately 410 lines vertical. Similarly, 1080i digital interlaced video (the "i" in 1080i refers to "interlaced") would need to be filtered to about 910 lines for an interlaced display, although a fixed pixel display (such as LCD television) eliminates the inaccuracies of scanning, and thus can achieve Kell factors as high as 95% or 1020 lines. It should be noted that the Kell Factor equally applies to progressive scan. Using a Kell factor of 0.9, a 1080p HDTV video system using a CCD camera and an LCD or plasma display will only have 1728 × 972 lines of resolution.

Fixed pixel array displays such as LCDs, plasmas, DLPs, LCoS, etc. need a "video scaling" processor with frame memory, which, depending on the processing system, effectively converts an incoming interlaced video signal into a progressive video signal. A similar process occurs in a PC and its display with interlaced video (e.g., from a TV tuner card). The downside is that interlace motion artifacts are almost impossible to remove resulting in horizontal "toothed" edges on moving objects.